Total syntheses of Nigrasin I and Kuwanon C

The efficient total syntheses of Nigrasin I and Kuwanon C, two biologically interesting natural flavonoids with two regioisomeric isoprenyl side chains, were realized for the first time starting from commercially available 1-(2,4,6-trihydroxyphenyl)ethanone via a linear reaction sequence of 11 steps with the overall yields of 22% and 21%, respectively, in which the selection of protective groups and the Claisen rearrangement of the allylated 16 featured the synthetic strategy. The use of acetic anhydride as the solvent and the employment of microwave irradiation are disclosed to be critically important in the efficient and selective Claisen rearrangement.